This invention relates to a method for producing an AB block copolymer with controlled polydispersity index (PDI) and molecular weight by reaction of at least one macroinitiator, capable of free radical polymerization, in the presence of at least one monomer and a compound of the formula (I).
Block copolymers exhibit unique properties and as a result are more desirable than random copolymers or a blend of the two homopolymers synthesized from the respective monomers. When block copolymers have a predetermined structure, molecular weight and molecular weight distribution, such properties become more attractive to new concepts developers for various industrial applications.
There are various commercial block copolymer products currently produced using ARTP, RAFT or TEMPO approaches. These “living”/controlled free radical polymerization technologies are expensive, complex, cause odor or impurities during the reaction. U.S. Pat. No. 6,204,319 provides an amphiphilic AB-block copolymer prepared via Group Transfer Polymerization (GTP) and uses the AB block copolymer in forming carbon black pigment dispersions for aqueous coating compositions with improved black color jetness. The disadvantages of GTP are high cost and only limited monomers (acrylates) can be used with the technology. U.S. Pat. No. 6,815,498 provides a method for preparation of block copolymer via 1,1-diphenyl ethylene (DPE) containing macroinitiator. Due to the high ceiling temperature of DPE copolymer, which is approaching the boiling point of an aqueous system, the method is impossible to be used in waterborne solutions. U.S. Pat. No. 7,439,292 prepares phosphate containing amphiphilic block copolymer via Reversible Addition Fragmentation Transfer polymerization (RAFT), as an additive for film-forming compositions on a metallic surface or to protect said metallic surface against corrosion. However, the odorous and expensive thiocarbonate chain transfer agents prevent the technology from being widely applied.
Above cost and odor problems were well solved by Chinese Patent CN 100554291C which uses α-methyl styrene (AMS) containing prepolymer as macroinitiator in subsequent polymerization. The macroinitiator is a polymeric chain containing AMS residue sites. AMS polymer has a ceiling temperature, at which temperature it is degraded to the monomer. At a temperature greater than 60° C., the polymeric chain breaks at the sites of AMS due to the low ceiling temperature (61° C.) of AMS and thus leading to the formation of polymeric radicals capable of the initiation of the second stage polymerization with living characteristics. The process is low cost in raw material and easily handled. It has potential to prepare block copolymers on an industrial scale. The reference mentions that the macroinitiator is suitable for synthesis of block copolymers, graft copolymers by melt blending with polypropylene or used as compatibilizer in polypropylene blends to reach an in situ compatibilization. This approach is believed feasible for large scale manufacture of block copolymers. However, due to the uncontrolled second stage initiated by the macroinitiator, the block copolymer products obtained from the reference have uncontrolled high weight average and number average molecular weight (Mn generally higher than 4.0×105 g/mol and Mw reaches 3.0×106 g/mol) and broad molecular weight distribution (polydispersity index (PDI) of from 2.0 to 4.0, which affords problems relating to high viscosity, low gloss or poor color strength when used as binders in coating formulations. Therefore, a preparation method with higher conversion rate, capable of control the PDI and the molecule weight of the block copolymer products is desired.